The present invention generally relates to a soldering interconnect method and apparatus for semiconductor units, and more particularly to a soldering interconnect system designed to secure multi-lead semiconductor packages to traces on a printed circuit board in a highly rapid and efficient manner.
Recent advances in microelectronic technology have resulted in the development of new, powerful, and compact integrated circuit packages. These packages typically incorporate a plurality of outer conductive leads designed for electrical attachment by soldering to an appropriate substrate (e.g. a printed circuit board). Attachment of the outer leads to the conductive traces on a printed circuit board requires special techniques to insure a high degree of accuracy and production efficiency. One type of circuit for which these considerations are especially important is a Tape Automated Bonded circuit package (hereinafter referred to as a "TAB" circuit).
TAB circuits were first researched and developed in the mid-1960's. As discussed in Rima, P.W., "The Basics of Tape Automated Bonding", Hybrid Circuit Technology, November, 1984, pps. 15-21, a TAB circuit is constructed using a thin film carrier tape which is typically stored on large reels. The tape has a variable width of between 8 and 70 mm, and is approximately 5 mils thick. The length of each portion of tape used to form an individual circuit package is selectively variable, depending on the type of circuit to be made. The tape may be manufactured from a variety of different dielectric materials including polyimide, polyester, and/or epoxy-glass compositions. Polyimide is preferred in that it has a high degree of mechanical strength, is capable of withstanding relatively high temperatures, and has a coefficient of linear expansion similar to that of copper. It also has a relatively low coefficient of moisture absorption (about 3%).
To construct the TAB circuit, an opening or "window" is punched through the center of each portion or "frame" of tape. Thereafter, a thin, conductive foil preferably manufactured of copper or copper alloy is bonded to the tape using an adhesive known in the art. The foil is approximately 1.4 mils thick, and is subsequently etched to produce a conductive printed circuit pattern having beam-type inner leads which extend into the window. The conductive printed circuit pattern terminates at the outer edges of the frame in a plurality of resilient beam-type outer leads which extend beyond the edges of the frame.
In an alternative construction method, the copper/copper alloy materials used to produce the circuit pattern may be directly sputtered or plated onto the tape. Thereafter, both the copper materials and polyimide are etched to the desired configuration. Next, an integrated circuit chip of selectively variable design and complexity is then positioned within the window and secured therein by bonding the inner leads to contact pads on the chip. This process (known as "inner-lead bonding") usually involves high speed soldering or thermocompression techniques known in the art. The resulting product consists of a thin-film panel with a conductive circuit pattern thereon having a center-mounted integrated circuit device. The entire unit is then positioned on a printed circuit board in order to secure the outer leads to conductive traces on the board in a process known as "outer lead bonding". At this stage in the fabrication process, great care must be taken in securing the outer leads of the TAB circuit to the printed circuit board. The large number of outer leads used in most TAB circuits combined with their small size and close spacing renders the outer lead bonding process difficult, time-consuming and subject to numerous inaccuracies.
The present invention involves an improved method and apparatus for bonding the outer leads of TAB circuits and other packaged semiconductor devices to conductive traces on a printed circuit board. The method and apparatus described herein is characterized by a high degree of accuracy, efficiency, and effectiveness. Accordingly, the present invention represents an advance in the art of soldering interconnect technology, as described in detail herein.